Add a zpos property to planes. Call drm_atomic_helper_check() instead of
calling drm_atomic_helper_check_modeset() and drm_atomic_check_planes()
manually. This effectively adds a call to drm_atoimic_normalize_zpos()
before checking planes. Reorder atomic update to allow changing plane
zpos without modeset.

Note that the initial zpos is set in ipu_plane_state_reset(). The
initial value set in ipu_plane_init() is just for show. The zpos
parameter of drm_plane_create_zpos_property() is ignored because
the newly created plane do not have state yet.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>